1. Field of the Invention
The present invention relates to a polymaleimide compound and a composition containing the same which are excellent in curing characteristics, and exhibit, when cured, a high heat resistance, a low linear expansion coefficient and good adhesion characteristics, and are easily processed.
2. Background
In the fields of electric and electronic apparatus and in transportation, such as air planes and cars, as the apparatuses have become sophisticated for high performance, more compact and lighter in weight, there has been an increasing demand for a material which has a high heat resistance, is excellent in adhesion properties and has a low linear expansion coefficient.
In the fields, epoxy resins, maleimide resins, of which the typical example is N,N'-4,4'-diphenylmethane bismaleimide, polyimide resins, and the like have heretofore been used.
However, conventional polyimide resins involve a difficult problem in that they are poor in moldability due to their being insoluble in suitable solvents and infusible, although the resins are excellent in heat resistance especially in terms of glass transition point (Tg).
With respect to maleimide resins which are thermocurable, the cured resins prepared therefrom exhibit a high heat resistance and have a low linear expansion coefficient but the maleimide resins are hardly soluble in low boiling point solvents such as methyl ethyl ketone, tetrahydrofuran and the like, so that the use of special high boiling point solvents is necessary for dissolving the resins. The use of such special high boiling point solvents to form solutions of the resins therefore presents various problems For example, not only a high temperature is required for drying and removing the solvent, a small amount of which still tends to remain even after drying, but also the remaining solvent has an unfavorable influence on the properties of the cured resin products. Moreover, the adhesion properties of the cured maleimide resins are extremely poor and, therefore, the resins present difficulties in practical use.
On the other hand, epoxy resins are soluble in a low boiling point solvent and excellent in mechanical and electrical properties, and exhibits good adhesion properties as compared with maleimide resins. However, the epoxy resins have the disadvantage that they are insufficient in heat resistance and high in linear expansion coefficient as compared with maleimide resins.
Hence there has been a continuing demand for a resin which is soluble in a low boiling point solvent, and exhibits, when cured, a high heat resistance, a low linear expansion coefficient and good adhesion properties. That is, a resin which has both of merits inherent in epoxy resins and maleimide resins respectively.
To attain the required properties, there has conventionally been proposed a blended resin, e.g. a blend of an epoxy resin and a maleimide resin. The blended resin is excellent in heat resistance. However, it is poor in adhesion property as compared with epoxy resins and it exhibits only a similar solvent-solubility to those of maleimide resins, giving no solution to the above-mentioned problem. It has also been attempted to modify maleimide resins with a view to improving a solvent-solubility and adhesion properties. For example, Japanese Patent Application Laid-Open Specification No. 52-121700/1977 discloses a reaction product of an .alpha.-arylmaleimide derivative with a compound having two or more epoxy groups. From the description of the laid-open specification No. 52-121700, it is presumed that the reaction product has a structure in which the epoxy resin skeleton has its terminals to which .alpha.-arylmaleimido groups are bonded. The reaction product can be easily photopolymerized through photodimerization of the .alpha.-arylmaleimido groups. The polymerization, however, gives only a solvent-insoluble polymer, which is still poor in heat resistance. If a Michael addition reaction is tried between the reaction product and a compound having active hydrogen atoms, the reaction would not proceed smoothly due to steric hindrance of the aryl group bonded to the maleimido group, so that there could not be obtained any crosslinked product having a high heat resistance.